The present invention relates to a photodetector for receiving an index signal of a beam-indexing type cathode-ray tube for use in a color television receiver, an image tube for a projector, etc.
A beam-indexing type flat color cathode-ray tube has been proposed in, for example, Japanese Patent Laid-Open Publication No. 74244/1986 as shown in FIG. 1. The known cathode-ray tube includes a flat glass tube 1, a deflection yoke 2, a light collector plate 3 and a panel 4 formed, on its inner surface, with a phosphor screen 5. The phosphor screen 5 is inclined relative to a central axis of an electron beam 6 when the electron beam 6 travels without deflection.
The phosphor screen 5 is formed as shown in FIG. 2. Namely, a layer 7 of black nonluminescent material such as carbon is formed in a shape of stripes on the inner surface of the panel 4. Index stripes 8 of a phosphor such as "P47" (name used in trade and manufactured by KASEI OPTONICS K.K. of Japan) are provided so as to interpose therebetween the layer 7. Phosphor stripes of primary colors of red (R), green (G) and black (B) are arranged at a predetermined interval on the layer 7 so as to have a definite relation with the index stripes 8. The index stripes 8 are disposed in some 9' of a plurality of gaps 9 between the phosphor stripes of the primary colors. The phosphor stripes of the primary colors have a thickness sufficient for saturating luminance of the phosphor stripes of the primary colors at a maximum luminance.
Meanwhile, the phosphor screen 5 is wholly covered with a transparent protective thin film 10 of silicon dioxide (SiO.sub.2) Furthermore, a very thin transparent electrically conductive film 11 such as a thin film of ITO (indium oxide doped with tin oxide) is formed on the thin film 10 by vacuum evaporation so as to wholly cover the phosphor screen 5.
By the above described arrangement of the known cathode-ray tube, since the phosphor stripes of the primary colors R, G and B and the index stripes 8 of the phosphor are directly excited by the electron beam 6, bright images can be observed through an observation window 12 and index light of high intensity can be obtained through the panel 4. Moreover, since the layer 7 of the black nonluminescent material is disposed in the gaps 9 except for the gaps 9' in which the index stripes 8 of the phosphor are positioned, contrast ratio of images can be improved.
Index light transmitted through the panel 4 is incident upon the light collector plate 3 where a wavelength of the index light is changed to that corresponding to sensitivity of a photoelectric conversion element. Then, the index light is converted into an electric signal by the photoelectric conversion element so as to be outputted as an index signal.
As shown in FIG. 3, the light collector plate 3 has a rectangular shape. Since such light collector plate 3 is already known from, for example, Japanese Patent Laid-Open Publication No. 121664/1979, detailed description thereof is abbreviated for the sake of brevity. The light collector plate 3 is obtained by dispersing an organic phosphor such as "Macro-lex Fluorescent Yellow 10GN" (name used in trade and manufactured by Bayer AG of West Germany), Rhodamine B, etc. in plastics such as acrylic resin or transparent support material such as glass. In FIG. 3, a photodiode 13 is attached to one side of the light collector plate 3.
In FIG. 4, horizontal scanning lines (scanning electron beams) for scanning the phosphor screen 5 in the known cathode-ray tube are shown by broken lines. When an upper side portion a, a central portion b and a lower side portion c of the phosphor screen 5 are scanned by an electron beam, the light collector plate 3 is disposed at the position shown by the one-dot chain lines. Thus, signal strength, namely, intensity of light received by the photodiode 13 is compared between a case in which the photodiode 13 is disposed at the position shown in FIG. 3 and a case in which two photodiodes 13 are attached to one side of the light collector plate 3 as shown in FIG. 5. Namely, FIGS. 6a, 6b and 6c show distributions of signal strength at the upper side portion a, the central portion b and the lower side portion c of the phosphor screen 5, respectively in the case where the photodiode 13 is attached to one side of the light collector plate 3 as shown in FIG. 3. Likewise, FIGS. 7a, 7b and 7c show distributions of signal strength at the upper side portion a, the central portion b and the lower side portion c of the phosphor screen 5, respectively in the case where the two photodiodes 13 are attached to one side of the light collector plate 3 as shown in FIG. 5. As will be seen from a signal strength S1 in FIG. 6a and a signal strength S2 in FIG. 7a, strength of signals applied to positions of the light collector plate 3, which positions correspond to opposite ends of the upper side portion a of the phosphor screen 5, becomes extremely small.
This reason is described with reference to FIGS. 8 and 9, hereinbelow. Namely, assuming that characters A and B denote phosphor particles excited by index light in the light collector plate 3 of FIG. 9, paths of light generated from the phosphor particles A and B are shown by the arrows in FIG. 9, respectively. Furthermore, supposing that character M denotes an area of a light receiving surface 13a of the photodiode 13 and character K denotes luminous intensity of the phosphor particles A and B, intensity LA of light of the phosphor particle A applied, at an angle .theta.1 of incidence, to the light receiving surface 13a of the photodiode 13 and intensity LB of light of the phosphor particle B applied, at an angle .theta.2 of incidence, to the light receiving surface 13a of the photodiode 13 are given as follows by setting the angles .theta.1 and .theta.2 of incidence at 10.degree. and 90.degree., respectively. EQU LA=KM sin.theta.1=0.17KM LB=KM sin.theta.2=KM
Thus, it will be understood that since light of the phosphor particles disposed in the vicinity of the opposite ends of the upper side portion a of the phosphor screen 5 is incident upon the light receiving surface 13a of the photodiode 13 at a small angle of incidence when the photodiodes 13 are arranged as shown in FIGS. 3 and 5, strength of the index signals at the opposite ends of the upper side portion of the light collector plate 3 becomes small.